I converted audio files to 16 bit rate and noise is gone.
"is playing" feature can be good. You can add in future. (Not urgent for me)
And I have a new question for you. I'm trying to add web fonts to game. I tried both Google Web Fonts and fonts in my web server. In the browser(Chrome) no problem it is working but in windows phone web fonts not working. I tried a lot of fonts and many different variations but results are same. Browser OK. Windows Phone not working. Do you have any idea ?